Nanocrystalline titanium alloy, and method and apparatus for manufacturing the same

ABSTRACT

A method and apparatus for manufacturing a nanocrystalline titanium alloy by performing an equal channel angular pressing process to a titanium alloy material, and a nanocrystalline titanium alloy manufactured using the method and apparatus. The method for manufacturing the nanocrystalline titanium alloy includes steps of preparing a titanium alloy material, and performing an equal channel angular pressing process on the titanium alloy material at an isothermal condition of 575° C. to 625° C. The nanocrystalline titanium alloy according to has a grain size of 300 nm.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean Patent Application Nos. 10-2004-0049406 filed on Jun. 29, 2004, and 10-2005-0007872 filed on Jan. 28, 2005, both applications filed in the Korean Intellectual Property Office, the entire content of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a nanocrystalline titanium alloy and a method and apparatus for manufacturing the same, and more particularly to a method and apparatus for manufacturing a nanocrystalline titanium alloy by performing an equal channel angular pressing process to a titanium alloy material, and a nanocrystalline titanium alloy manufactured by using the method.

(b) Description of the Related Art

Since a titanium alloy has a high specific strength and excellent corrosion resistance, it can be widely used in various fields such as in the aerospace industry, the chemical industry, as an implant material, and as a sports product material. Since the titanium alloy has an improved superplastic property, weight and manufacturing cost thereof can be reduced by a superplastic forming process. Accordingly, if the titanium alloy is applied to various industries, significant benefits can be obtained.

It is known that a titanium alloy must be subjected to the superplastic forming process at a high process temperature of 850° C. or more and a slow process rate of 10⁻³/sec or less. However, since the superplastic property is significantly affected by microstructure, a titanium alloy consisting of fine grains can be subjected to the superplastic forming process at a lower process temperature and a quicker process rate. Thereby, as the nano-technology is developed, research into a method for manufacturing a titanium alloy having fine grains has been actively progressed.

On the other hand, a method for manufacturing a material having fine grains includes a powder metallurgy method, a mechanical alloying method, a rapid solidifying method, a recrystallization method, a forging method, a rolling method, and a drawing method. However, it is difficult to manufacture a material having a desired size using these methods, and internal pores may be formed in the material. Since, the size of the recrystallization grain is limited or the cross section is reduced by the increment of the deformation amount. Thus, a large amount of deformation cannot be applied to the material. Accordingly, there is a limit to refining the grain size of the material. Thus, these methods for refining the grain cannot be actually applied.

Recently, rigid-plastic working methods for performing plastic working with separate heat treatment and refining a grain in which pores are not formed have been suggested. The rigid-plastic working methods include a high pressure torsion (HPT) method, an equal channel angular pressing (ECAP) method, and so on.

The HPT method shear-deforms a material at a high pressure and can be performed at a rapid rate even at room temperature. However, there is a limit in the size of the material, and the microstructure and the thickness of the material are inhomogeneous.

The ECAP method is a method for introducing a material into an L-shaped channel and shear-deforming the material. The ECAP method can be performed using existing press equipment. Also, the ECAP method can be scaled up and thus is economical. In addition, although the deformation amount increases, the cross section of the material does not decrease and thus a large amount of deformation can be applied to the material.

In case of titanium alloys, however, the process temperature of the titanium alloy is very high and the flow stress thereof is reduced as the deformation amount increases. Thus, extreme cracks may be generated in the surface of the titanium alloy when performing the ECAP method. Accordingly, it is difficult to manufacture a titanium alloy having nanometer-size grains by the ECAP method.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a nanocrystalline titanium alloy which consists of fine grains without cracks and that can be subjected to a superplastic forming process, and a method for manufacturing the same.

Moreover, another object of the present invention is to provide an apparatus for manufacturing a nanocrystalline titanium alloy by an adequate process condition.

The present invention provides a method for manufacturing a nanocrystalline titanium alloy including steps of preparing a titanium alloy material and performing an ECAP (equal channel angular pressing) process to the titanium alloy material at an isothermal condition of 575° C. to 625° C. The nanocrystalline titanium alloy is manufactured by the method for manufacturing the nanocrystalline titanium alloy according to the present invention and may have a grain size of 300 nm or less.

The present invention also provides an apparatus for manufacturing a nanocrystalline titanium alloy including an ECAP unit having a bent channel, a temperature holding unit which surrounds the ECAP unit and includes at least one heating member for heating the ECAP unit to a predetermined temperature, and a temperature measuring unit for measuring the temperature of the ECAP unit.

As mentioned above, according to the method for manufacturing the nanocrystalline titanium alloy of the present invention, the ECAP process is performed in an optimal range. Thus cracks are not generated and the nanocrystalline titanium alloy having the nanometer-size grain can be manufactured. At this time, since a secondary process or coating agent which was conventionally required is not used, the process can be simplified and a nanocrystalline titanium alloy having a larger volume can be easily manufactured.

Further, the nanocrystalline titanium alloy according to the present invention has nanometer-size grains, and it has extremely excellent elongation because the segmented beta phases are uniformly distributed in the entire microstructure of the alloy. Particularly, a nanocrystalline titanium alloy of a lamellar structure which has low elongation and thus cannot be conventionally used can have the highest elongation. Thereby, the titanium alloy material used in the ECAP process can be variously selected, and thus the application fields of the nanocrystalline titanium alloy become various.

Moreover, by using the superplastic forming temperature lower than the conventional temperature, the manufacturing cost of the superplastic forming process can be remarkably reduced, and problems such as abrasion of the process apparatus at a high temperature and a reduction of the life span can be reduced, and the advent of impurities can be solved.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings, in which:

FIG. 1 is a schematic diagram of an apparatus for manufacturing a nanocrystalline titanium alloy according to an embodiment of the present invention;

FIG. 2 is a schematic perspective view of an ECAP unit shown in FIG. 1;

FIG. 3 is a plan view of the apparatus shown in FIG. 1;

FIG. 4 is a cross-sectional view schematically showing an operating principle of the apparatus shown in FIG. 1;

FIG. 5 is a flowchart showing a method for manufacturing a nanocrystalline titanium alloy according to an embodiment of the present invention;

FIG. 6 is a perspective view schematically showing an ECAP processing step of the present invention;

FIGS. 7A to 7D are photographs of nanocrystalline titanium alloys of Embodiment 1 to Embodiment 4 of the present invention;

FIG. 8 is a photograph of a titanium alloy of Comparative Example 1;

FIG. 9 is a photograph of a nanocrystalline titanium alloy of Embodiment 5 of the present invention;

FIGS. 10A to 10C are photographs of initial microstructures of titanium alloy materials used in Embodiment 6 to Embodiment 8 of the present invention taken with an optical microscope;

FIGS. 11A to 11C are photographs of nanocrystalline titanium alloys manufactured in Embodiment 6 to Embodiment 8 of the present invention taken with an optical microscope and a scanning electron microscope;

FIGS. 12A to 12C are photographs of nanocrystalline titanium alloys manufactured in Embodiment 6 to Embodiment 8 of the present invention taken with a transmission electron microscope;

FIG. 13A is a photograph of the nanocrystalline titanium alloy manufactured in Embodiment 6 after heat treatment at 600° C. taken with a transmission electron microscope;

FIG. 13B is a photograph of the nanocrystalline titanium alloy manufactured in Embodiment 6 after heat treatment of 650° C. taken with a transmission electron microscope;

FIG. 13C is a photograph of the nanocrystalline titanium alloy manufactured in Embodiment 6 after heat treatment of 700° C. taken with a transmission electron microscope;

FIG. 14 is a graph showing flow stress curves of nanocrystalline titanium alloys manufactured in Embodiment 6 to Embodiment 8 of the present invention;

FIG. 15 is a photograph of initial sample pieces of the nanocrystalline titanium alloys manufactured in one of Embodiment 6 to Embodiment 8 and sample pieces thereof after a tensile test;

FIG. 16 is a photograph of an initial sample piece of a titanium alloy manufactured by a conventional high pressure torsion (HPT) method and a sample piece thereof after a tensile test; and

FIG. 17 is a photograph of an initial sample piece of a titanium alloy manufactured by a conventional thermomechanical treatment and a sample piece thereof after a tensile test.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, a nanocrystalline titanium alloy and an apparatus and method for manufacturing a nanocrystalline titanium alloy according to the present invention will be described in detail by explaining exemplary embodiments of the invention with reference to the attached drawings.

The apparatus for manufacturing the nanocrystalline titanium alloy according to the present invention will first be described in detail, and then the method for manufacturing the nanocrystalline titanium alloy and the nanocrystalline titanium alloy will be described in detail.

FIG. 1 is a schematic diagram of an apparatus for manufacturing a nanocrystalline titanium alloy according to an embodiment of the present invention, FIG. 2 is a schematic perspective view of an ECAP unit shown in FIG. 1, and FIG. 3 is a plan view of the apparatus shown in FIG. 1.

Referring to FIGS. 1 to 3, the apparatus according to the embodiment of the present invention is fixed to a hydraulic press device (not shown) by a fastening member 50, and includes an ECAP unit 10, a temperature measuring unit 20, a temperature holding unit 30, and an adiabatic unit 40.

The ECAP unit 10 can be made by adhering two blocks, and includes an L-shaped channel 12 having a uniform cross section. An effective strain of a one-time ECAP process can be adjusted according the bending degree of the channel 12. By repeatedly performing the ECAP process, the total effective strain increases in multiple proportions. In the present embodiment, an effective strain of 1 can be applied to a material by a one-time ECAP process using an ECAP unit 10 of which the inner contact angle (θ of FIG. 4) of the bent portion of the channel 12 is 90° and the outer arc angle (ψ of FIG. 4) is 40°.

The temperature measuring unit 20 is located on one side of the ECAP unit 10 and measures a temperature of the ECAP unit 10. In the present embodiment, the temperature measuring unit 20 may be, for example, a thermoelectric couple.

The temperature holding unit 30 is located at the periphery of the ECAP unit 10 and holds the temperature of the ECAP unit 10 to be uniform. In the present embodiment, the temperature holding unit 30 may be formed in a cylindrical shape, and the diameter of the lower portion 30 b may be greater than that of the upper portion 30 a thereof.

Also, the temperature holding unit 30 includes a cylindrical heating member 33 for heating the ECAP unit 10 to hold a uniform temperature. The heating member 33 is connected to an external power source (not shown) and receives power for holding the temperature of the ECAP unit 10. In the present embodiment, the heating member 33 includes eight inner heating members 33 a which are located adjacent to the ECAP unit 10, and four outer heating members 33 b which are located at the outer portion of the temperature holding unit 30. Here, the inner heating members 33 a control the temperature of the ECAP unit 10 and the outer heating members 33 b insulate it.

Although the temperature, holding unit 30 includes twelve cylindrical heating members 33 in the present embodiment, the present invention is not limited to this. Various structures for uniformly holding the temperature of the ECAP unit may be applied and may be included in the scope of the present invention.

Also, the adiabatic unit 40 is formed on the lower surface of the temperature holding unit 30 and minimizes the heat transfer to the outside. The adiabatic unit 40 may be composed of asbestos.

FIG. 4 is a cross-sectional view schematically showing an operating principle of the apparatus shown in FIG. 1.

Referring to FIG. 4, the apparatus for manufacturing the nanocrystalline titanium alloy according to the present embodiment is fixed to the hydraulic press device (not shown) having a plunger 14 by a fastening member 50, for example, a fixing screw. At this time, by changing the rate of the plunger 14, the process rate can be controlled.

In the present embodiment, a titanium alloy material 16 is injected into the channel 12 of the ECAP unit 10 and is then pressed by the plunger 14 so that the titanium alloy material 16 passes through the bent portion of the channel 12, thereby performing the ECAP process. In the present specification, the titanium alloy material 16 is a titanium alloy which is not subjected to all ECAP processes, and the nanocrystalline titanium alloy is a titanium alloy which is subjected to all ECAP processes.

The titanium alloy material 16 is shear-deformed when passing through the bent portion of the channel 12 by the ECAP process. In the ECAP apparatus, since the cross section is not changed when performing the process, a very large amount of deformation can be applied to the material. Accordingly, large amount of deformation energy can be accumulated in the material and the accumulated deformation energy serves as a driving power for refining the grain.

The apparatus for manufacturing the nanocrystalline titanium alloy according to the present embodiment can uniformly hold the temperature of the ECAP unit by including the temperature measuring unit 20, the temperature holding unit 30, and the adiabatic unit 40. Thus, an adequate process condition can be applied to the material when performing the ECAP process.

FIG. 5 is a flowchart showing a method for manufacturing a nanocrystalline titanium alloy according to an embodiment of the present invention, and FIG. 6 is a perspective view schematically showing an ECAP processing step of the present invention.

Referring to FIG. 5, the method for manufacturing the nanocrystalline titanium alloy according to the present embodiment includes a preparing step S11 for a titanium alloy material, a preheating step S12 for preheating the titanium alloy material, and an ECAP processing step S13 for performing the ECAP process to the titanium alloy material at an isothermal condition of 575° C. to 625° C.

Further describing the method in detail, first, a titanium alloy material is prepared (S11). At this time, a titanium alloy material having various compositions and shapes may be prepared in consideration of an application field of the titanium alloy.

In the present embodiment, an example of a titanium alloy material comprises titanium as a main material and aluminum at 6 weight %, vanadium at 4 weight %, and other impurities. The aluminum and vanadium are added to increase the strength and ductility. Here, the amount of the aluminum is determined to prevent Ti₃Al from being formed, which may weaken the material upon the shear deformation, and the amount of the vanadium is determined to prevent a flaking phenomenon which may be caused upon cooling. The titanium alloy material having these compositions has excellent strength at a high temperature and good formability and thus can be applied to various fields.

Also, the titanium alloy material is formed of mixtures of alpha(α) phases and beta(β) phases. And, an initial microstructure of the titanium alloy is an equiaxed crystal structure or a lamellar structure in which a beta phase is formed between the alpha phases in a thin band shape. The equiaxed crystal structure may be formed by heating at the region in which the alpha phase and the beta phase are mixed and then cooling. The lamellar structure may be formed by heating at a temperature greater than a transformation temperature of the beta phase and then cooling by using nucleation and an auto-catalytic growth mechanism. The size of colony of the lamellar structure can be controlled by the time of the heating treatment, and the interlayer interval of the lamellar structure can be controlled by the cooling rate.

At this time, in order to minimize problems caused due to friction of the titanium alloy material and the channel of the ECAP unit during the ECAP process and more equalize a process rate, graphite may be coated on the titanium alloy material.

Next, the titanium alloy material is preheated to the isothermal condition of 575° C. to 625° C. for 7 minutes 30 seconds to 12 minutes 30 seconds (S12). This temperature is equal to the ECAP process temperature. That is, this temperature allows the temperature of the inside of the titanium alloy material to be uniform during performance of the ECAP process to more improve the effect due to the isothermal condition.

The apparatus according to the present invention can control the process temperature and hold the isothermal condition. Accordingly, in the present embodiment, the preheating step can be performed by the apparatus according to the present invention. Thereby, the process can be simplified and the preheating effect can be more efficiently realized. However, the present invention is not limited to this. The titanium alloy material may be preheated using a separate device or process and this is also included in the scope of the present invention.

Subsequently, the ECAP process is performed to the titanium alloy material in the isothermal condition of 575° C. to 625° C. (S13). If the process temperature of the ECAP process is greater than 625° C., the ECAP apparatus may be deformed or the grain of the titanium alloy material may be grown. Also, if the process temperature of the ECAP process is less than 575° C., it is difficult to process because the titanium alloy material is a high-temperature material and cracks may be generated therein. That is, the temperature condition of the ECAP process is determined to an optimal temperature to efficiently refine the grain of the titanium alloy and to not generate cracks.

At this time, the process rate of the ECAP process may be 0.4 mm/s to 2 mm/s. If the process rate of the ECAP process is greater than 2 mm/s, cracks may be generated by the repetitive ECAP process, and if the process rate of the ECAP process is less than 0.4 mm/s, the ECAP processing time is very long and thus the process efficiency may be deteriorated. That is, the process rate of the present invention is determined to a condition that does not generate cracks in the titanium alloy material and can optimize the process time of the ECAP process. At this time, it is preferable that the process rate of the ECAP process is in the range of 1.3 mm/s to 2 mm/s.

Furthermore, the total effective strain of the ECAP process may be in the range of 1 to 8. In order to apply an adequate total effective strain, the ECAP process may be repeatedly performed one or more times.

Referring to FIG. 6, if the ECAP process is repeatedly performed twice or more times, from the second ECAP process the titanium alloy material 16 rotates by a predetermined angle centering around a central axis L passing through a center C of an inlet 121 of the channel 12.

In the present embodiment, the rotation angle of the titanium alloy material 16 may be substantially 180° in each ECAP process.

That is, in a first ECAP process, the titanium alloy material 16 is injected into the channel 12 so that any virtual point A which exists in the titanium alloy material 16 passes through an outer bent portion 12 b, as shown in (a) of FIG. 6. In a second ECAP process, the titanium alloy material 16 is injected into the channel 12 so that the virtual point A passes through an inner bent portion 12 a, as shown in (b) of FIG. 6.

By this process, the titanium alloy material 16 deformed by the first ECAP process is deformed to an original shape after the second ECAP process. At this time, the deformation is concentrated to one shear surface by the first ECAP process and then the deformation is performed again by the second ECAP process. Thereby, a nanocrystalline titanium alloy of the equiaxed crystal is obtained by the even number of ECAP processes.

In the present invention, the rotation angle of the titanium alloy material 16 and the number of the ECAP processes can be variously adjusted.

In the method for manufacturing the nanocrystalline titanium alloy according to the present invention, a separate upset forging step or the usage of a coating agent is unnecessary by holding the factors of the ECAP process in an adequate range. Thus, the effective of the process can be improved.

In the nanocrystalline titanium alloy according to the present invention manufactured using this method, the grain size is in range of 300 nm or less and few cracks are formed by performing the ECAP process at the isothermal condition of an adequate temperature.

Also, by rotation of the titanium alloy material when performing the ECAP process, the nanometer-size grain can be formed even in the case that the initial microstructure of the titanium alloy material is the lamellar structure. Also, the nanocrystalline titanium alloy according to the present invention can have a high dislocation density associated with the grain refining.

At this time, in the nanocrystalline titanium alloy according to the present invention, the beta phases are segmented and are uniformly formed in the entire microstructure to increase the boundary between the alpha phase and the beta phase in the alloy. Generally, since the boundary sliding of the boundary between the alpha phase and the beta phase is superior to that of the boundary between the alpha phases and the boundary between the beta phases, the nanocrystalline titanium alloy according to the present invention can have greater elongation.

The segmented beta phases have different characteristics according to the initial microstructure of the titanium alloy material. That is, the beta phases may be more segmented in the titanium alloy material of a lamellar structure having a predetermined lamellar spacing, so the lamellar structure having a predetermined lamellar spacing may have a very high elongation characteristic because of low stress. That is, in the present invention, a nanocrystalline titanium alloy having much better elongation can be manufactured using the titanium alloy material of the lamellar structure, which cannot be conventionally used because it has low elongation. Thereby, in the ECAP process, the titanium alloy material may be variously selected.

Generally, if the strain-rate sensitivity exponent is equal to or less than 0.45, it is known that the nanocrystalline titanium alloy cannot have the superplastic property. However, the nanocrystalline titanium alloy according to the present invention has excellent elongation of at least 300% although the strain-rate sensitivity exponent is equal to or less than 0.4. Accordingly, the nanocrystalline titanium alloy according to the present invention has an excellent superplastic property, which is improved by three times to eight times with respect to the titanium alloy having the micrometer-size grain. This is because the growth of the neck is deteriorated due to the work-hardening phenomenon according to the grain refining, and the boundary between the alpha phase and the beta phase is well-formed and thus the boundary sliding is easy. Thereby, the application field of the nanocrystalline titanium alloy can become various.

Also, the nanocrystalline titanium alloy according to the present invention is thermally stable at the temperature of 575° C. to 725° C., and the superplastic forming can be performed at this temperature. That is, in the nanocrystalline titanium alloy according to the present invention, the grain is suppressed from being coarsened by the recrystallization and the grain growth as the temperature increases.

In the nanocrystalline titanium alloy according to the present invention, the superplastic forming temperature is in the range of 575° C. to 725° C., which is less than the conventional superplastic forming temperature by 150° C. to 300° C. Thereby, in the nanocrystalline titanium alloy according to the present invention, problems such as abrasion of the apparatus at a high temperature and reduction of the life span can be reduced. As the result, the cost of the superplastic forming can be reduced.

Hereinafter, the nanocrystalline titanium alloy according to the present invention will be described in detail through experiments. The below-mentioned embodiments are exemplary and the present invention is not limited to these. Here, as an alloy material, a titanium alloy containing aluminum at 6 weight % and vanadium at 4 weight % was used.

<Experiment 1>

Embodiment 1

A titanium alloy material having a diameter of 9.5 mm and a length of 80 mm was subjected to the ECAP process once at an isothermal condition of 600° C. and a process rate of 7.3 mm/s to 10 mm/s to manufacture the nanocrystalline titanium alloy according to Embodiment 1.

Embodiment 2

A titanium alloy material having a diameter of 9.5 mm and a length of 80 mm was subjected to the ECAP process once at an isothermal condition of 600° C. and a process rate of 3.2 mm/s to 4.2 mm/s to manufacture the nanocrystalline titanium alloy according to Embodiment 2.

Embodiment 3

A titanium alloy material having a diameter of 9.5 mm and a length of 80 mm was subjected to the ECAP process once at an isothermal condition of 600° C. and a process rate of 1.3 mm/s to 2 mm/s to manufacture the nanocrystalline titanium alloy according to Embodiment 3.

Embodiment 4

A titanium alloy material having a diameter of 9.5 mm and a length of 80 mm was subjected to the ECAP process once at an isothermal condition of 600° C. and a process rate of 0.4 mm/s to 0.44 mm/s to manufacture the nanocrystalline titanium alloy according to Embodiment 4.

COMPARATIVE EXAMPLE 1

A titanium alloy material having rectangular of 23.75 mm and 23.01 mm, and length of 127 mm was subjected to the ECAP process at a non-isothermal condition to manufacture a titanium alloy according to Comparative Example 1. The non-isothermal condition means that the titanium alloy material is held at 900° C. for 45 minutes to reduce the flow stress and is then subjected to the ECAP process at 300° C. for a short time of 2 seconds.

The photographs of the surfaces of the nanocrystalline titanium alloys according to Embodiment 1 to Embodiment 4 are shown in FIGS. 7A to 7D, respectively, and the effective strain, the surface crack depth, and the crack fraction of the nanocrystalline titanium alloys according to Embodiment 1 to Embodiment 4 are shown in Table 1. Additionally, a photograph of the surface of the nanocrystalline titanium alloy according to Comparative Example 1 is shown in FIG. 8. TABLE 1 Shear Surface Process defor- crack Crack rate Process mation Effective depth fraction [mm/s] time [s] time [s] strain [mm] [%] Embodiment 7.3-10  11-8 0.7 1.5 3.0 32.6 1 Embodiment 3.2-4.2  25-19 1.3 0.75 1.5 16.5 2 Embodiment 1.3-2  54-40 3.3 0.3 0.5 5.4 3 Embodiment 0.4- 200- 16.6 0.06 0.4 4 4 0.44 179

Comparing FIGS. 7A to 7D with FIG. 8, the nanocrystalline titanium alloys according to Embodiment 1 to Embodiment 4 have a surface crack depth of 3 mm or less, but the titanium alloy of Comparative Example 1 has a surface crack depth of 10 mm or more. That is, it can be seen that excessive cracks are generated in the nanocrystalline titanium alloy of Comparative Example 1. Also, the nanocrystalline titanium alloys according to Embodiment 1 to Embodiment 4 have crack fractions of at most 32.6%, but the titanium alloy of Comparative Example 1 has at least 50%.

That is, it can be seen that an excellent nanocrystalline titanium alloy can be manufactured by performing the ECAP process at an isothermal condition of 575° C. to 625° C. according to the present invention. Thereby, the production rate of the nanocrystalline titanium alloys according to Embodiment 1 to Embodiment 4 increases to more than that of Comparative Example 1 by at least 70% in consideration of the size thereof. That is, it can be seen that the nanocrystalline titanium alloy according to the present invention can improve productivity.

If the crack fraction is greater than 10%, fine cracks generated in the first process may provide the location of the cracks which may be generated in the second process. Accordingly, it is preferable that the crack fraction is 10% or less. Accordingly, it is preferable that the process rate is equal to or less than 2 mm/s.

Also, if the process rate is less than 0.4 mm/s, the process time exceeds 200 seconds. Accordingly, it is preferable that the process rate is 0.4 mm/s or more in view of process efficiency.

<Experiment 2>

Embodiment 5

A titanium alloy material having a diameter of 9.5 mm and a length of 80 mm was subjected to the ECAP process four times at an isothermal condition of 600° C. and a process rate of 1.3 mm/s to 2 mm/s to manufacture the nanocrystalline titanium alloy according to Embodiment 5. A photograph of the surface of the nanocrystalline titanium alloy according to Embodiment 5 is shown in FIG. 9.

Referring to FIG. 9, it can be seen that the nanocrystalline titanium alloy according to Embodiment 5 is composed of uniform grains having sizes of 300 mm or less. This is because the grain is refined by the ECAP process for applying the adequate process rate and the strain at the isothermal condition of an adequate temperature. That is, the nanocrystalline titanium alloy having uniform grains having sizes of 300 mm or less can be manufactured.

<Experiment 3>

Embodiment 6

A titanium alloy material was subjected to a heating treatment at a temperature of 950° C. for 2 hours and then a furnace cooling treatment to prepare a titanium alloy material of an equiaxed crystal structure having a grain size of 11 μm. An optical microscopic photograph of this titanium alloy material is shown in FIG. 10A.

This titanium alloy material was subjected to the ECAP process four times at a temperature of 600° C. to manufacture the nanocrystalline titanium alloy according to Embodiment 6. At this time, the rotating angle of each ECAP process was 180°.

Embodiment 7

A titanium alloy material was subjected to a heating treatment at a temperature of 1050° C. for 1 hour and then a furnace cooling treatment to prepare a titanium alloy material of a lamellar structure having a colony size of 310 μm and an interlayer interval of 4.1 μm. An optical microscopic photograph of this titanium alloy material is shown in FIG. 10B.

This titanium alloy material was subjected to the ECAP process four times at a temperature of 600° C. to manufacture the nanocrystalline titanium alloy according to Embodiment 6. At this time, the rotating angle of each ECAP process was 180°.

Embodiment 8

A titanium alloy material was subjected to a heating treatment at a temperature of 1050° C. for 1 hour and then a furnace cooling treatment to prepare a titanium alloy material of a lamellar structure having a colony size of 310 μm and an interlayer interval of 1 μm. An optical microscopic photograph of this titanium alloy material is shown in FIG. 10C.

This titanium alloy material was subjected to the ECAP process four times at a temperature of 600° C. to manufacture the nanocrystalline titanium alloy according to Embodiment 8. At this time, the rotating angle of each ECAP process was 180°.

The nanocrystalline titanium alloys according to Embodiment 6 to Embodiment 8 were photographed with an optical microscope and a scanning electron microscope and the photographs are shown in FIGS. 11A to 11C, respectively, and were photographed with a transmission electron microscope and are shown in FIGS. 12A to 12C. In FIGS. 12A to 12C, zone axis is ^([{overscore (1)}2{overscore (1)}3]).

Referring to FIGS. 11A to 11C, the grain sizes of the alpha phase and the beta phase of the nanocrystalline titanium alloy according to the present invention are smaller than those of the titanium alloy materials shown in FIGS. 10A to 10C. At this time, although the equiaxed crystal and the lamellar structure have different strengths and deformation behavior, the grain can be refined by the manufacturing method according to the present invention.

Also, in the nanocrystalline titanium alloys according to Embodiment 6 to Embodiment 8, the beta phases are extremely deformed and segmented and thus are uniformly distributed in the entire microstructure. Particularly, it can be seen that the beta phases are most extremely segmented in the nanocrystalline titanium alloy according to Embodiment 8.

Accordingly, the nanocrystalline titanium alloys according to Embodiment 6 to Embodiment 8 have high elongation by the boundary sliding of the boundary between the alpha phase and the beta phase. It can be seen that the nanocrystalline titanium alloy according to Embodiment 8 in which the beta phases are most extremely segmented has the highest elongation.

Referring to FIGS. 12A to 12C, it can be seen that the grain sizes of the nanocrystalline titanium alloys according to Embodiment 6 to Embodiment 8 are in the range of 200 nm to 300 nm, and the dislocation density is very high and the grain boundary is not obvious. That is, according to the nanocrystalline titanium alloy according to the present invention, the nanocrystalline titanium alloy having fine grains and high dislocation density can be manufactured.

<Experiment 4>

Photographs of the nanocrystalline titanium alloy according to Embodiment 6 which was subjected to the heating treatment at 600° C., 650° C., and 700° C., respectively, are shown in FIGS. 13A, 13B, and 13C, respectively.

As shown in FIGS. 13A to 13C, it can be seen that the grain is fine and the dislocation density is high after the nanocrystalline titanium alloy according to Embodiment 6 was subjected to the heating treatment of 600° C., 650° C., and 700° C., respectively. That is, it can be seen that the nanocrystalline titanium alloy according to the present invention is thermally stable at the above temperature. This tendency is shown in Embodiment 7 and Embodiment 8. That is, it can be seen that the nanocrystalline titanium alloy according to the present invention is not coarsened even at the temperature of 575° C. to 725° C. and has a fine grain.

<Experiment 5>

The nanocrystalline titanium alloys according to Embodiment 6 to Embodiment 8 were subjected to a tensile test at a temperature of 700° C. for 10⁻³/sec. The measured flow stress curve is shown in FIG. 14. Also, the maximum elongations of the titanium alloy materials of Embodiment 6 to Embodiment 8, and the maximum elongation, the strain-rate sensitivity exponent, and the work-hardening exponent of the nanocrystalline titanium alloy according to Embodiment 6 to Embodiment 8 are shown in Table 2.

Initial sample pieces (i) of the nanocrystalline titanium alloys according to one of Embodiment 6 to Embodiment 8 and sample pieces (a) of the nanocrystalline titanium alloys which were subjected to a tensile test at a temperature of 700° C. for 10⁻³/sec, and sample pieces (b) of the nanocrystalline titanium alloys which were subjected to a tensile test at a temperature of 700° C. for 10⁻⁴/sec are shown in FIG. 15.

Further, an initial sample piece (i) of the titanium alloy manufactured by a conventional HPT process, a sample piece (a) of the alloy which is subjected to a tensile test at a temperature of 650° C. for 10⁻²/sec, a sample piece (b) of the alloy which was subjected to a tensile test at a temperature of 650° C. for 10⁻⁴/sec, and a sample piece (c) of the alloy which was subjected to a tensile test at a temperature of 725° C. for 10⁻²/sec are shown in FIG. 16. Moreover, an initial sample piece (i) of the titanium alloy manufactured by a conventional thermomechanical treatment and a sample piece (a) of the titanium alloy which was subjected to a tensile test at a temperature of 800° C. for 10⁻²/sec, a sample piece (b) of the titanium alloy which was subjected to a tensile test at a temperature of 800° C. for 10⁻³/sec, and a sample piece (c) of the titanium alloy which was subjected to the tensile test at a temperature of 800° C. for 2×10⁻²/sec are shown in FIG. 17. TABLE 2 Embodiment Embodiment Embodiment 6 7 8 Maximum elongation of 163 96 78 titanium alloy material [%] Maximum elongation of 476 330 610 nanocrystalline titanium alloy [%] Strain-rate sensitivity 0.33 0.30 0.36 exponent of nanocrystalline titanium alloy Work-hardening exponent 0.62 0.65 0.80 of nanocrystalline titanium alloy

Referring to FIG. 14, it can be seen that the work-hardening phenomenon in which stress increases as the strain increases to a certain strain occurs in the nanocrystalline titanium alloys according to Embodiment 6 to Embodiment 8. Also, it can be seen that the nanocrystalline titanium alloys according to Embodiment 6 to Embodiment 8 have very excellent superplastic properties. The flow stress increases in the order of Embodiment 7, Embodiment 8, and Embodiment 6.

Referring to Table 2 and FIG. 15, it can be seen that the nanocrystalline titanium alloys according to Embodiment 6 to Embodiment 8 have high elongation of 300% or more although the strain-rate sensitivity exponents are 0.33, 0.30, and 0.36, respectively. This is because local neck growth is suppressed by the work-hardening phenomenon. It can be seen that the work-hardening exponent increases in the order of Embodiment 6, Embodiment 7, and Embodiment 8.

Particularly, it can be seen that an elongation of 610% can be obtained by performing the ECAP process according to the present invention to the titanium alloy material of the lamellar structure having the elongation of 78% in Embodiment 8. Conventionally, the titanium alloy material having the lamellar structure cannot be used, because it has low elongation. However, the nanocrystalline titanium alloy manufactured by the manufacturing method according to the present invention can have higher elongation compared with the nanocrystalline titanium alloy having the equiaxed structure. This is because the nanocrystalline titanium alloy according to Embodiment 8 has a highest work-hardening exponent and the beta phases are uniformly segmented in the entire microstructure by the ECAP process as in FIG. 11C of experiment 3 and thus the boundary between the alpha phase and the beta phase is much formed. On the other hand, referring to FIG. 16, the titanium alloy manufactured by the conventional HPT process may have high elongation. However, it cannot be seen that the elongation is accurately measured, because the titanium alloy was manufactured from a very small titanium alloy material. Further, the HPT process cannot be applied to actual industry, because the microstructure according to the diameter direction of the sample piece is very inhomogeneous and the size of the titanium alloy which can be processed is very small. Also, the elongation result according to the thermomechanical treatment shown in FIG. 17 is measured at a high temperature of 800° C., and this result cannot be obtained at a low temperature as in the present invention.

That is, the titanium alloy according to the present invention has an excellent superplastic property at a process temperature lower than that of the conventional titanium alloy, and a nanocrystalline titanium alloy with a size that can be used in the actual industry can be manufactured.

Although the exemplary embodiments of the present invention have been described, the present invention is not limited to the exemplary embodiments, but may be modified in various forms without departing from the scope of the appended claims, the detailed description, and the accompanying drawings of the present invention. Therefore, it is natural that such modifications belong to the scope of the present invention. 

1. A method for manufacturing a nanocrystalline titanium alloy comprising steps of: preparing a titanium alloy material; and performing an ECAP (equal channel angular pressing) process to the titanium alloy material at an isothermal condition of 575° C. to 625° C.
 2. The method of claim 1, wherein the ECAP process has a process rate of 0.4 mm/s to 2 mm/s.
 3. The method of claim 2, wherein the ECAP process has a process rate of 1.3 mm/s to 2 mm/s.
 4. The method of claim 1, wherein a total effective strain of the ECAP process is 1 to
 8. 5. The method of claim 1, comprising performing the ECAP process at least twice.
 6. The method of claim 5, comprising rotating the titanium alloy material by a predetermined rotation angle with respect to a previous ECAP process centering around a central axis passing through a center of an inlet of the channel, from a second ECAP process.
 7. The method of claim 6, wherein the rotation angle is substantially 180°.
 8. The method of claim 7, comprising performing the ECAP process an even number of times.
 9. The method of claim 1, further comprising preheating the titanium alloy material at a temperature of 575° C. to 625° C. for 7 minutes 30 seconds to 12 minutes 30 seconds, between the preparing of the titanium alloy material and the performing of the ECAP process.
 10. The method of claim 1, wherein the titanium alloy material comprises titanium as a main material and aluminum at 6 weight %, vanadium at 4 weight %, and other impurities.
 11. The method of claim 10, wherein an initial microstructure of the titanium alloy material is an equiaxed crystal structure or a lamellar structure.
 12. A nanocrystalline titanium alloy manufactured by the method of claim 1, wherein the nanocrystalline has a grain size of 300 nm or less.
 13. The nanocrystalline titanium alloy of claim 12, wherein the nanocrystalline titanium alloy comprises a mixture of alpha phases and beta phases and the beta phases are segmented and distributed in the entire microstructure of the nanocrystalline titanium alloy.
 14. The nanocrystalline titanium alloy of claim 13, wherein the nanocrystalline titanium alloy has a maximum elongation of 300% or more.
 15. The nanocrystalline titanium alloy of claim 13, wherein the nanocrystalline titanium alloy has a strain-rate sensitivity exponent of 0.4 or less.
 16. The nanocrystalline titanium alloy of claim 13, wherein the nanocrystalline titanium alloy has a superplastic forming temperature of 575° C. to 725° C.
 17. An apparatus for manufacturing a nanocrystalline titanium alloy, comprising: an ECAP unit including a bent channel; a temperature holding unit surrounding the ECAP unit and including at least one heating member for heating the ECAP unit to a predetermined temperature; and a temperature measuring unit for measuring the temperature of the ECAP unit.
 18. The apparatus of claim 17, further comprising an adiabatic unit provided on a lower portion of the temperature holding unit.
 19. The apparatus of claim 18, wherein the adiabatic unit comprises asbestos.
 20. The apparatus of claim 17, wherein an inner contact angle of the bent portion of the channel is 90° and the outer arc angle thereof is 40°. 